A Detailed Analysis of the Operational Orbit of the Hipparcos Satellite

An analysis of the orbital evolution of the ESA's Hipparcos satellite is presented. Hipparcos operated between August 1989 and March 1993 in a highly elliptical orbit: a geostationary transfer orbit with increased perigee height. The requirements of the scientific mission included high accuracy knowledge of the position and velocity vectors of the spacecraft as a function of time. Through a study of the variations in the total orbital energy, the loss of energy during the mission as a result of non-conservative forces is recovered. These are explained as largely due to atmospheric drag during perigee passages. Apparent variations in the drag coefficient are in agreement with orientation variations of the satellite during those perigee passages. Two different models used for calculating the atmospheric drag give significantly different results, confirming earlier findings by other users of those models. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modelling the Torques Affecting the Hipparcos Satellite

Reconstructed attitude data for the Hipparcos mission as obtained in the final stages of the data analysis for the published catalogue is used to derive detailed information on the dynamics of the satellite. Most elements of the inertia tensor of the satellite could be calibrated from the observed acceleration data, which are also used to reconstruct torques due to solar radiation and gravity gradient, and the magnetic moment of the satellite and it's interaction with the magnetic field surounding the Earth. The effects of the oblateness of the Earth on the gravity gradient are evaluated and shown to be negligable. The magnetic field model includes both the `main' and the `disturbance' fields. The remaining systematic effects in residual torques are most likely attributed to variations in the magnetic field that are local and are beyond the models used to describe it. The angular momentum vector for one of the gyros was reconstructed from the torque it asserted on the satellite while it was running in redundant mode. This revised version was published online in August 2006 with corrections to the Cover Date.     

Introduction to a Further Examination of the Hipparcos Data

We present an introduction to four papers on further analysis of the raw Hipparcos data. This analysis has lead to the recognition of how orbit, radiation and temperature conditions did or didn't affect the scientific results of the Hipparcos mission. It also led the way to a new reduction of the scientific data that shows from its initial results a real potential for a substantial improvement of the astrometric parameters for stars brighter than V=8.5. This short paper serves as an introduction to the four main papers, and provides some general references. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Operational Environment of the Hipparcos Mission

We investigate links between the observational environment as experienced by the Hipparcos satellite and the performance of the spacecraft and payload instrumentation, with particular emphasis on finding out whether some of these effects may have been inadequately represented in instrument calibrations and could thus have affected the scientific results of the mission. Scan-coverage and radiation effects are primarily random effects with only some long-term systematics. However, long- (days to weeks) and short-term (hours) temperature variations reflected in the performance of some of the spacecraft instrumentation. It is shown that only a small sign of some long-term thermal variations could be detected in the payload instrumentation. These findings further limit the scope left for the occurrence of large-scale correlated errors in the Hipparcos astrometric data. On the other hand, a number of great circles were identified which showed a highly significant drift of the basic angle, which had not been detected in the preparation of the published data. The data from these circles may have, in some cases, led to, very localised, slightly anomalous results, in particular where stars are accidentally affected by two or more of such circles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dynamic Modelling of the Hipparcos Attitude

We present a new method for a high-accuracy reconstruction of the attitude for a slowly spinning satellite. This method, referred to as the fully-dynamic approach, explores the possibility to describe the satellite's attitude as that of a rigid body subject to continuous external torques. The method is tried out on the Hipparcos data and is shown to reduce the noise for the along-scan attitude reconstruction for that mission by about a factor two to three. The dynamic modelling is expected to give a more accurate representation of the satellite's attitude than was obtained with a pure mathematical modelling. As such, it decreases the degrees of freedom in the a posteriori reconstruction. Some of the decrease is obtained through accumulating and subsequently implementing information on high frequency components in the solar radiation torques, which show to be systematic and predictable. This could be expected, as they are primarily linked to the external geometry and optical properties of the satellite. In the context of an astrometric mission, the methods presented here can only be applied as a final iteration step: the star positions that are used to reconstruct the attitude are also part of the scientific objectives of the mission. An estimate for the potential of a re-reduction of the Hipparcos data using the fully-dynamic model for the attitude reconstruction was obtained from test reductions of the first 24 months of mission data. Improvement of the accuracies of the astrometric parameters for all stars brighter than Hp=9.0 appears possible. The noise on the astrometric parameters for these stars was affected significantly by the along-scan attitude noise, which dominated for stars brighter than Hp=4.5. The possible improvement for stars brighter than about Hp=4.5 may, after iterations, be as much as a factor three. The reduced noise levels also allow a more accurate calibration and monitoring of instrument parameters, leading potentially to a better understanding of the instrument and the scientific data obtained with it. This revised version was published online in August 2006 with corrections to the Cover Date.